The wide-spread use of low-loss and low-dispersion single-mode optical fibers in the 0.8-1.6 .mu.m wavelength region demands an efficient means of coupling power between optical fibers and optical devices, especially between a semiconductor laser or amplifier and the optical fiber. For example, an increase in the coupling efficiency and, thus, a decrease in the coupling loss between the laser and the fiber, permits increase in the repeater spacing in long-haul and submarine transmission systems. However, efficient coupling of semiconductor lasers to optical fiber has been a problem of general concern since the advent of optical fiber communications. Since the laser light power that can be launched into a single mode fiber suffers a loss of 7-11 dB via butt-joint coupling, coupling efficiency is universally improved either by the use of a microlens on the end of the fiber to match the modes of the laser and optical fiber or by bulk optics or by a combination of microlenses and bulk optics. Microlenses are more commonly used because of their ease of fabrication and packaging.
For communication, microlenses are very often used as parts of a communication package comprising an optical fiber and an optical device, such as a semiconductor laser at amplifier, optical fiber amplifier, optical fiber amplifier, or a pump for a fiber amplifier. The optical communication package may take many configurations and embodiments. One of these is shown in an article by G-D. Khoe et al. "Laser Monomode-Fiber Coupling" and Encapsulation in a Modified To-5 Package", Journal of Lightwave Technology, Vol. LT-3, No. 6, December 1985 pp. 1315-1320. Another example is disclosed in an article by J.-I Minowa et al., "Optical Componetry Utilized in Field Trail of Single Mode Fiber Long-Haul Transmission, IEEE Journal of Quantum Vol. QE-18, No. 4, April, 1982, pp. 705-717.
Microlenses are typically fabricated by tapering the fiber down to a point and melting the end. The tapering may be effected either by etching an end portion of the fiber in acid or by heating a section of the fiber and pulling-apart the heated section. The heating may be executed with a flame, an electric arc or a laser. The resultant microlenses are hemispherical in shape and, unfortunately, consistently demonstrate imperfect coupling, collecting typically less than 50 percent (-3 dB), and at best 55 percent (-2.5 dB) of the available laser radiation. The remainder of the light from the laser is lost.
U.S. application Ser. No. 07/333,230, filed Apr. 5, 1989 (Presby, H. M., Case 39) (U.S. Pat. No. 4,932,989), which is incorporated herein by reference, discloses a novel, laser-machining technique for producing arbitrarily shaped microlenses at the end of an optical fiber. While the production of microlenses by this technique is simplified and expedited, relative to the production of such microlenses by tapering technique, these microlenses still exhibit a relatively high coupling loss (e.g. 1.5-4.5 dB, see FIG. 5 of the patent application).
W. Bludau and R. H. Bossberg, in "Low-Loss Laser-to-Fiber Coupling with Negligible Optical Feedback", Journal of Lightwave Technology, vol. LT-3, No. 2, April 1985, pp. 294-302, describe an attempt to improve the laser-to-fiber coupling efficiency with simultaneous reduction in optical power feedback by changing the shape of the microlens from a hemispherical to aspherical (hyperbolic) form. The microlens was produced by a cumbersome multistep process that includes splicing a short length (about 1 mm) of a large diameter silica rod (d=240 .mu.m) to a monomode fiber (core diameter typically 10 .mu.m), heating the free end of the silica rod to produce a hemispherical lens with a diameter (d=355 .mu.m) larger than the original diameter of the silica rod, tipping the center of the hemisphere with a droplet of pure quartz, and remelting the lens so that the droplet merges into the lens body with resulting lens shape approximating an aspherical lens shape. However, while this design led to the reduction in optical feedback, the improvement in coupling efficiency was not sufficiently advantageous, with coupling efficiency amounting to .gtoreq.40 percent, with the best value being only 70 percent. Therefore, it is still desirable to increase the coupling efficiency between an optical device and an optical fiber above and beyond the prior art results. Also, the microlenses should be reproducibly fabricated in a simple and expeditious way.